Procesos costeros.pdf

CHAPTER 15 Hydrodynamics and Coastal Processes

Summary of key outcomes:

Numerical modelling techniques have been used to assess potential changes to the wave climate, currents and stability of coastal sediment on a Bay-wide basis as a result of the proposed Port Botany Expansion.

Wave climate studies examined swell waves, wind generated local sea waves and long waves. Swell waves and local sea waves are the primary wave types which affect sediment transport and shoreline stability in Botany Bay.

The results of the wave modelling showed that beyond the northeastern embayment of Botany Bay (i.e. between the Parallel Runway and Molineux Point) wave climate in Botany Bay would be effectively unchanged as a result of the construction of the new terminal. There would be a minor reduction in sediment transport on Towra Beach, however, any changes would be imperceptible. There would also be a small change in wave direction, in the order of 1o at Silver Beach. However, this would be accommodated within the existing groynes hence there would be no identifiable impact.

The results of the modelling also showed that currents in Botany Bay would be essentially unchanged as a result of the construction of the new terminal. Subsequent detailed modelling and analysis therefore focused on changes in wave climate in the northeastern embayment between the Parallel Runway and Molineux Point.

Within the northeastern embayment, potential changes to swell wave energy, notably along the Parallel Runway, have been minimised by careful design of the dredge area and the new terminal. With the proposed dredging profile and rock embankment design for the new terminal, there would be no increase in swell wave energy on the eastern side of the Parallel Runway and local sea wave heights along the Parallel Runway would be reduced.

A large reduction in wave energy would be expected at the existing mouth of Penrhyn Estuary due to the “blocking effect” of the new terminal. Construction of the new terminal would therefore significantly reduce erosion and transportation of sediment along the southeast portion of Foreshore Beach. There would continue to be westward sand transport along the remaining exposed section of Foreshore Beach. However, a groyne would be built as an extension to the Mill Stream training wall to trap this sediment and prevent it from blocking the Mill Stream waterway.

Port Botany Expansion Environmental Impact Statement – Volume 1


15 Hydrodynamics and Coastal Processes

15.1 Introduction

The study of water movement, predominantly caused by tides and wind is termed “hydrodynamics”. Consideration of hydrodynamic issues is important when major development is proposed in estuarine or coastal settings, particularly as coastal developments may result in changes to wave climate which can impact areas remote from the development site.

Botany Bay is a wide, shallow estuary exposed to winds from all directions and waves from the adjacent high energy coastal zone. Waves and currents determine the erosion, deposition, transportation and therefore, the ultimate fate of sediment in the Bay. Changes to the Bay hydrodynamic conditions can affect transportation of sediment around the Bay and hence impact on the stability of beaches and infrastructure. Previous developments, most notably in northern Botany Bay, have modified the foreshores and substantially changed the hydrodynamics of the Bay.

This chapter describes the existing hydrodynamics and coastal processes in Botany Bay and assesses any impacts of the proposed Port Botany Expansion. It is based on investigations undertaken by Lawson and Treloar (2003) in a report titled Proposed Expansion of Container Port Facilities in Botany Bay, NSW – Coastal Processes and Water Resources Issues: Volume 3 Waves, Currents and Coastal Process Investigations. The results of the investigations are provided in Appendix H.

15.2 Existing Conditions and Processes

The entrance to Botany Bay is about 1.1 km wide and is open to ocean swell from the east. Despite the exposure to waves and a small tidal range, tidal processes are the predominant influence of circulation and flushing in Botany Bay. Botany Bay is relatively shallow (mean water depth is about 5 m) and shoals westward from the dredged navigation channel at the entrance. The foreshores of Botany Bay include rocky headlands, sandy beaches and sheltered embayments. Numerous developments on the foreshore of Botany Bay have modified the hydrodynamics and shoreline of Botany Bay.

The major riverine input to the Bay is from the Georges River in the southwest corner of the Bay. The smaller Cooks River discharges to the northwestern corner of the Bay, west of Sydney Airport. The fresh water flow from both rivers is normally small compared to the tidal flow.

Developments such as the Caltex Oil Refinery, Parallel Runway and Port Botany have extensively modified Botany Bay. The Caltex Oil Refinery was established at Kurnell in 1953 and the approach channel and jetty berths were dredged and extended in several stages.

Construction of Sydney Airport was completed in several stages and involved the relocation of the Cooks River and Alexandra Canal west of their original courses. The north-south runway of Sydney Airport was extended into Botany Bay in two stages (1964-66 and 1970-71) using sand dredged from Botany Bay southwest of the runway and from the entrance of the Bay. The Parallel Runway was constructed in 1994 from sand extracted from the Bay to the east of the runway.

Port Botany was constructed between 1978 and 1982. As part of the construction, an armoured revetment wall extending south from Bumborah Point was built and dredging of a ship navigation channel from the entrance of the Bay to the port was undertaken.

The construction of Sydney Airport and Botany Bay has created hard edged structures within the Bay.

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Changes to Bay hydrodynamics have impacted on sediment transport and beach stability around the Bay. Consequently seawalls have been constructed on Lady Robinsons Beach, and Silver Beach and southern Lady Robinsons Beach have been stabilised by groynes.

15.2.1 Tides

The tidal regime in Botany Bay is essentially the same as in Port Jackson with a mean tidal period of 12.4 hours. Generally two high tides and two low tides occur each day (semi-diurnal) and are almost simultaneous throughout the Bay. The tidal amplitude varies fortnightly on a high and low range tidal cycle (spring and neap tides, respectively) and the maximum and minimum heights of each successive tide vary significantly. The range of astronomical tides is approximately 2.1 m, mean high to mean low water spring tides is some 1.3 m and mean high to mean low water neaps is around 0.8 m.

15.2.2 Currents and Circulation

Currents in Botany Bay are predominantly generated by tides, winds and river flow. Irregular, long period waves originating offshore (e.g. storm surges) are capable of generating currents. However, the current velocities are low (less than 2 cm/s in Port Botany) due to the low amplitude and long period. During low rainfall periods, currents in the Bay are dominated by tidal flow to and from the Georges River.

Tidal currents that produce a periodic flow in and out of Botany Bay are generally low in velocity. The highest velocity tidal currents occur at the entrance to the Bay, at the mouth of the Georges River, near Molineux Point and around the southern edge of the Parallel Runway. Only minor alterations to the tidal current flow and direction have resulted from existing infrastructure as the major (east west) flow is south of the Parallel Runway (i.e. in the middle of the Bay). Maximum tidal velocities are likely to cause only local resuspension of the sandy sediment that predominates throughout the Bay. Tidal currents in the northeastern embayment of Botany Bay are generally low (less than 0.1 m/s) due to the confined nature of the northeastern embayment, but higher velocity flows occur near the Mill Stream.

Wind blowing across a water body generates water currents by transference of momentum. Current velocities are dependent on strength, fetch and duration of wind. Botany Bay is exposed to winds over large stretches of open water (fetch) and wind driven currents assist in exchange and mixing of estuarine waters.

Botany Bay is stratified immediately after heavy rainfall (greater than 50 mm per day) as turbid, buoyant freshwater plumes exit the estuary mouth (Kingsford 1994). The freshwater plumes are some 1-2 m thick and have little interaction with underlying saline waters. After the freshwater flow ceases, the surficial layer deepens due to wind mixing and the estuary reverts to low-flow conditions after a week of dry conditions.

15.2.3 Wave Climate

Two principal wave types occur within Botany Bay, wind generated local sea waves and ocean waves (swell waves) generated offshore that pass through the entrance to Botany Bay. Three wave classifications, i.e. local sea, swell and long waves, have been considered throughout Botany Bay as part of this assessment.

Waves generated by wind blowing over restricted areas of open water are referred to as local sea waves. Due to the open expanse of water in Botany Bay, waves can be generated by winds from all points of the compass. Local sea waves can be important in controlling circulation and near-shore resuspension and

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transportation of sediment. Water depth has little effect on the generation of local sea waves, but increases in water depth (e.g. from dredging) can change the shoaling of waves and sediment transportation patterns.

Diurnal northeast winds (~8 m/s) during summer produce waves with periods typically about 2.5 seconds that affect Towra and Silver Beaches on the southern foreshores of Botany Bay. The Sydney Airport runways have limited the fetch of easterly winds to Lady Robinsons Beach on the western foreshores of Botany Bay. Strong southerly winds (~10 m/s), particularly late on summer afternoons, are usually of short duration, but can produce wind waves that temporarily influence water circulation in northern Botany Bay. Westerly winds, which can persist for up to a week at a time in winter, have long fetches in Botany Bay, but the waves generated in deeper water near the mouth of Botany Bay are probably insignificant in terms of resuspension of sediment.

Swell refers to waves generated by distance disturbances, e.g. storms and cyclones, that have longer wavelengths and periods than local sea waves. Botany Bay is subject to ocean swell from the east, however, the swell wave climate near the new terminal area is described as “mild” since energy from incoming waves is redirected by entrance dredging, or obstructed by the armoured revetment wall north of Molineux Point. The swell wave climate has substantially changed with developments in the Bay, which has contributed to erosion and accretion problems in some areas.

Long waves (wave periods generally between 25 and 300 seconds) have a variety of sources including distant storms, changes in atmospheric pressure and wave grouping. Long waves have the potential to cause excessive movement of moored vessels, but disruptions from long waves have not been reported in Botany Bay. The long wave climate in Botany Bay interpreted from nearly 5 months of wave data from Waverider buoys has been described as “very mild” and horizontal currents generated from long waves are less than 0.02 m/s.

15.2.4 Coastal Processes

Coastal processes in Botany Bay include the erosion, transportation and accumulation of sediment and therefore the shape and nature of beaches. The stability of sandy beaches on the western and southern shoreline of Botany Bay is influenced by ocean swell waves that penetrate the entrance to the Bay. Dredging for navigational purposes at the entrance to Botany Bay has substantially changed the wave climate and therefore erosional and depositional areas in the Bay. To a lesser extent, locally generated wind waves and currents also influence sediment transport and beach stability in Botany Bay.

Silver and Towra Beaches on the southern shoreline of the Bay are subject to ocean swell waves through the entrance of Botany Bay. These sandy beaches represent dynamic sedimentary environments as the beaches are not aligned parallel to the impinging waves. Storm erosion on Silver Beach has been reduced by groynes placed on the beach in 1969-70, 1980 and 1992. In contrast, existing westward sand transportation along the unprotected Towra beach is estimated at about 6,000 m3 per annum.

Ocean swell has been almost completely excluded from the northern foreshores of Botany Bay due to the existing terminals at Port Botany and the Sydney Airport runways. Tidal currents are also low and sediment is accumulating in previously dredged areas. Sand is accumulating along the northern section of the beach from northerly transport of sediment.

Muddy embayments (e.g. Woolooware and Quibray Bays) are low energy, depositional environments on the southern shoreline of Botany Bay and are largely sheltered from wave effects.



Foreshore Beach is presently subject to moderate wave energies, particularly during strong southerly winds. Migration of sand (i.e. longshore drift toward the Mill Stream outlet) is causing erosion along Foreshore Beach and recession of beach dunes. A comparison of recent photographs with those taken in 1996 also indicates that sand has accumulated in outer Penrhyn Estuary, in the form of a spit northwest of Brotherson Dock. Sand accumulation in the outer sections of Penrhyn Estuary has reduced access to the Estuary to a narrow “neck” and necessitated relocation of the boat ramp in Penrhyn Estuary.

The existing dredged area between Port Botany and the Parallel Runway is a low energy environment.

15.3 Methodology

Investigations were undertaken by numerical computer modelling and included assessment of wave climate, currents and coastal processes. Potential changes as a result of the proposed Port Botany Expansion were assessed on a Bay-wide basis. Detailed assessments were also undertaken within the northeastern embayment of Botany Bay between the Parallel Runway and Molineux Point.

Modelling included the following:

waves - two modelling systems were used to determine the extent of wave climate change (at mean sea level) in Botany Bay. The SWAN wave model was used to investigate the propagation of swell into Botany Bay from the Tasman Sea and the generation of wind generated local sea waves within the Bay. The MIKE-21 Boussinesq Wave (BW) model was used to model swell wave and long wave propagation in the Port Botany area between the Parallel Runway and Molineux Point; and

currents - two models were developed both using the Delft 3D modelling system - a whole of Bay model extending up the Georges River to Lugarno, and a northern Bay regional model extending north from a line between the Parallel Runway and Molineux Point (Figure 15.1); and

the stability of coastal sediment in Botany Bay was assessed using historical information and the results from the wave and current modelling. In addition, the LITPACK coastal processes model was applied to Towra Beach and Foreshore Beach to determine potential impacts on sediment transport rates in these areas.

The modelling systems are described in more detail in the following sections.

15.3.1 Modelling of Waves

As outlined above, two numerical modelling systems were used to investigate present and post development wave conditions in Botany Bay.

The SWAN wave model was used to investigate the propagation of swell into Botany Bay from the Tasman Sea and the generation of local sea within the Bay. The model was set up using a constant grid size of 50 m throughout the Bay, from the 100 m depth contour offshore to Sandringham Bay on the northern side of the Georges River entrance.

For swell wave investigations, waves from nine offshore directions from north through east to south, combined with nine wave periods from 3 to 11 seconds were modelled, totalling 81 basic cases. In each case, the offshore wave height adopted was 1.5 m. To investigate changes in swell wave conditions along the Bay shorelines, 67 locations around the Bay were selected for model output for the SWAN wave model.



figure 15.1



The SWAN wave model was also used to investigate potential changes in the local sea or wind generated wave conditions within Botany Bay. Wave simulations were undertaken for varying wind speeds from 0 m/s to 25 m/s (in increments of 2.5 m/s) for all directions at increments of 22.5°. For local sea waves, 60 years of Mascot wind data was applied to the SWAN model results.

The MIKE-21 BW model was used to model swell waves propagating into the port area between the Parallel Runway and Molineux Point. The model was set up with a grid size of 4 m for swell waves and operated with a time step of 0.25 seconds. The investigation included modelling of both a rock embankment wall (the preferred design for the new terminal) and a fully reflective vertical wall.

The long wave model set up was also based on the MIKE-21 BW system, but with a 12 m grid size and simulations over 30 minutes to ensure equilibrium was established.

The wave models were calibrated to ensure that the modelling system would accurately reproduce swell and local sea wave conditions in Botany Bay. This was achieved using measured data collected by the Waverider buoys at the locations shown in Figure 15.1.

The models were firstly calibrated using data from the Waverider buoys at the Entrance Channel (WRB5), Towra Beach (WRB9), Lady Robinsons Beach (WRB10) and in the middle of the Bay (WRB3) with wave length and period data from Sydney Ports Corporation’s offshore Botany Bay Waverider buoy, and wave direction from Manly Hydraulics Laboratory’s Long Reef Waverider buoy. The second calibration was undertaken investigating propagation of waves at locations near the Parallel Runway (WRB16) and Molineux Point (MID) and similar offshore data to the first calibration exercise. The outcomes of the calibration analyses for the combined SWAN/MIKE-21 BW modelling system provided a high level of confidence in the modelling results.

Several months of wave data recorded by Sydney Ports Corporation from the Waverider buoy near the Parallel Runway (WRB16) and wind data from the Mascot Airport anemometer were available for calibration of the SWAN wave model for local sea conditions. The modelled and measured wave heights were compared and the agreement was found to be very good.

It is important to note the modelling has been undertaken on a comparative basis. In other words, the model focused on the differences between wave conditions for existing and post-development cases. Therefore, any discrepancy in model performance would be similar for both cases and would not influence the assessment of possible changes due to the proposed Port Botany Expansion.

15.3.2 Modelling of Currents

The potential changes to currents within Botany Bay were examined using numerical modelling methods. Two models were developed: a whole of Bay model and a northern Bay regional model extending north from the line between the Parallel Runway and Molineux Point. Both models were based on the Delft3d modelling system that has been used extensively for major national and international projects.

The whole of Bay model was developed using a grid structure generally consistent with the major tidal flows in the Bay. The model was driven using tides, winds and stormwater inflows into the Bay. However, this model could not resolve the small changes proposed in the Port Botany area therefore the northern Bay regional flow model was applied to the detailed investigation of local issues in the Port Botany area.



Tidal predictions in Botany Bay are generally similar to those in Sydney Harbour and tidal constants at Fort Denison (Sydney Harbour) were used in modelling to predict currents. Wind data was taken from recorded Mascot airport anemometer data, as required. Catchment discharge data, where needed, was prepared using data from hydrological investigations (Appendix I).

Calibration of the models was undertaken to provide confidence that the modelling systems can realistically reproduce observed current structure. While it is difficult to reproduce current structure near the entrance to Botany Bay, the outcome of these calibration processes demonstrates that the models provide a generally reliable description of currents within Botany Bay. Similar to the wave modelling, the current modelling was undertaken on a comparative basis and therefore any discrepancy in model performance would be the same for both the existing and post-development cases.

15.4 Assessment of Impacts

Changes to wave climate, currents and sediment transport attributed to the new terminal are likely to be subtle, complex and interrelated. The following post constructional changes to hydrodynamic conditions in Botany Bay have been considered:

• local sea wave conditions throughout Botany Bay, along the Parallel Runway wall, within the northeastern embayment and within Brotherson Dock;

• swell wave conditions throughout Botany Bay, along the Parallel Runway wall, within the northeastern embayment and within Brotherson Dock;

• long waves in Botany Bay, along the Parallel Runway wall, within the northeastern embayment and within Brotherson Dock;

• stability of foreshores in Botany Bay and the northeastern embayment; and

• currents in Botany Bay, along Foreshore Beach and at the Mill Stream outlet.

15.4.1 Wave Conditions

Local Sea Waves

Assessment of local sea waves showed that negligible changes to existing conditions would occur in parts of Botany Bay outside the immediate Port Botany area. The fetch of northerly winds (Port Botany to Kurnell) would be reduced by the new terminal. No change to wave heights would be expected, however, about a one degree westward change in local sea wave direction is predicted at Silver Beach and Towra Beach. This may cause a minor change in erosion and deposition of sand, however, no overall impact on beach stability is predicted as sediment transport resulting from local sea waves is small compared to that caused by swell waves. Additionally, the groynes on Silver Beach would accommodate any change. The net result on Towra Beach would be a marginal reduction in the rate of westward sediment transportation, however, such a reduction would be imperceptible.

A large reduction in wave energy would be expected at the existing mouth of Penrhyn Estuary due to the “blocking effect” of the new terminal. Construction of the new terminal would therefore significantly reduce erosion and transportation of sediment along the southeast portion of Foreshore Beach and the current



accumulation of sand in outer Penrhyn Estuary. There would continue to be westward sand transport along the remaining exposed section of Foreshore Beach. However, a groyne to be built as an extension to the Mill Stream training wall would act as a barrier to this transport and prevent sand from accumulating in the Mill Stream. There may be a need to remove sand collected by the groyne every four to five years and replace it near the boat ramp.

The wave height at the proposed new boat ramp area is suitable for a boat ramp.

A reduction in the significant height of local sea waves would be expected along the Parallel Runway due to the shorter fetch created by construction of the new terminal. Local sea conditions would not cause sediment transport along the eastern side of the Parallel Runway due to the depths being too great, therefore changes in wave direction are not an issue. No increase in local sea wave conditions is predicted in Brotherson Dock. Local sea wave conditions at the proposed new berths are predicted to be very mild.

Swell Waves

Possible changes to wave climate due to the new terminal were assessed at 67 locations around the perimeter of Botany Bay. Output from the SWAN model indicated that no changes to swell wave height would occur throughout the Bay. A change in wave direction of less than 0.1o was identified at Towra Beach. However, this change is at the limit of capability of any model to simulate and essentially describes a no-change scenario. This change would not be measurable against the natural regression rate in this area.

Numerical modelling investigated the impact on the Parallel Runway of berth designs using rock revetment or vertical walls. Smaller changes in wave energy were predicted for designs incorporating rock revetments, which is the preferred design for the new terminal. The proposed port development with rock revetment and dredging would marginally reduce wave heights along the eastern side of the Parallel Runway.

Existing wave heights within the Brotherson Dock are low (less than 0.11 m), however the MIKE 21 BW model indicated an increase in wave height due to increased swell wave penetration after construction of the new terminal. The resultant wave heights are still very low therefore the post development swell wave climate would not affect berthed ships or existing port facilities.

Long Waves

Berthed ships are susceptible to movement from long waves and the resultant motion can affect the loading and unloading of vessels. Results of modelling indicate that the long wave climate throughout Botany Bay would not be changed by the proposed development, with the exception of minor amplification and increase in long wave activity within Brotherson Dock. Results of the modelling indicate that although current speeds generated by long waves are predicted to increase by about 10% in Brotherson Dock, they would remain below 2 cm/s. The low predicted velocities would be unlikely to cause significant disturbance to loading/unloading activities of berthed ships.

There would be no change in long wave activity along the Parallel Runway.

Vessel Operations

Waves generated by ships (wake) would not be expected to substantially change the wave climate in Port Botany and would not affect the stability of shorelines and existing structures.



Wake from tug operations would be unlikely to increase the overall wave climate on nearby Foreshore Beach. Similarly, propeller wash from tugs manoeuvring ships would not affect the stability of sediment adjacent to the Parallel Runway. A rock groyne is proposed to protect recreational vessels using the boat ramp from tug propeller wash.

15.4.2 Currents

Reclamation for the new terminal would result in a minimal decrease (~0.7%) in the total tidal prism of Botany Bay. In addition, the new terminal is proposed in the northern embayment of Botany Bay and would not obstruct the predominant tidal flow between the entrance of the Bay and Georges River. Therefore, the direction and speed of tidal currents in Botany Bay would not change significantly as a result of the new terminal. This conclusion is supported by modelling of changes to peak velocities at the southern end of the Parallel Runway that indicate the new terminal would not significantly change current velocities at that location.

Existing tidal current velocities in Port Botany are low (less than 10 cm/s) and a possible slowing of these currents due to changes to the tidal prism would not have an obvious effect on sediment transportation in the port area. There would be virtually no change in current speed or direction and therefore sediment transportation in northeastern Botany Bay, including the Mill Stream outlet, as result of the Port Botany Expansion.

No significant changes to wind generated surface currents from the new terminal would be expected due to the small changes predicted for significant wave heights and directions.

15.4.3 Coastal Processes

Botany Bay

Changes to coastal processes in Botany Bay due to the new terminal would be negligible for the following reasons:

the change to the tidal prism in Botany Bay is minor (decreased by some 0.7%);

circulation in Botany Bay would not be obstructed or reduced;

the reclamation would not impede tidal currents to and from the Georges River;

modification to water depths in high energy environments (i.e. the entrance to Botany Bay) would not be required; and

the new terminal would be constructed in a low energy environment sheltered from swell waves by Molineux Point.

Changes to the fetch of local sea waves by construction of the new terminal would result in small changes in wave directions on the southern foreshore of Botany Bay. However, the magnitude of these changes on coastal processes would be small compared to the effects of swell propagating through the entrance to Botany Bay. Changes to coastal processes resulting from the development would be predominantly confined to the northeastern embayment of Botany Bay.



Northeastern Botany Bay

Under existing hydrodynamic conditions, the eastern section of Foreshore Beach is undergoing long term recession and sand is accumulating at the western end of the beach near the Mill Stream outlet. The proposed development would reduce wave energies and hence regression rates at the present mouth of Penrhyn Estuary, but coastal processes at the Mill Stream end of Foreshore Beach would be essentially unchanged.

The Mill Stream presently discharges to northeast Botany Bay through a 40 m wide lined channel between the Parallel Runway and Foreshore Road. Part of the proposed port expansion includes an enhanced beach area to be established within a compartment formed by a southeastward extension of the northeastern Mill Stream training wall and the proposed boat ramp at the western end of the flushing channel from Penrhyn Estuary. The extended Mill Stream training wall provides an expansion of the Mill Stream Entrance, thereby leading to reduced currents in the lower Mill Stream. Assessment of changes to the current structure shows that there is a continuous flow from the combined flows from the Mill Stream and drains on Foreshore Beach. At low tide, water depth in this area is only about 1.4 m and peak current speeds presently reach 0.7 m/s. However, the velocities decrease rapidly with distance from the mouth of the Mill Stream and approximately 300 m from the Mill Stream, adjacent to the Parallel Runway, current velocities are much lower and well below speeds that can cause sediment movement.

The western section of Foreshore Beach (approximately 500 m in length) is to be rehabilitated and maintained between the proposed boat ramp and the proposed extension to the northern wall of the Mill Stream outlet. The local sea wave climate for this region would be very mild, but there would be continuing westward sand transport. A groyne about 25 m in effective length is proposed as part of an extended northern training wall to the Mill Stream. This structure would trap the longshore transport of sediment and prevent its accumulation in the Mill Stream. Hence, there is likely to be a continuing need to undertake beach maintenance by removing sand as it is transported to the groyne and deposit this near the boat ramp every four to five years.

Present accumulation of fine grained sediment in Penrhyn Estuary is low, probably due to a small influx of material from the catchment. In addition, fine grained sediment is presently resuspended by wave action at the mouth of Penrhyn Estuary. Due to the lower energy regime of Penrhyn Estuary following construction of the new terminal, sedimentation in the larger confined area of the Estuary would occur during low-flow conditions.

Catchment modelling indicates that during high rainfall events, peak flow into Penrhyn Estuary from Springvale and Floodvale Drains would exceed 30 m3/s. The resultant peak flow velocities near the constriction in Penrhyn Estuary are predicted to reach 0.26 m/s (5 years ARI) (refer to Location C in Figure 16.2) which is similar to the present modelled velocity of 0.30 m/s. The peak flow velocities during some high rainfall events may be sufficient to scour and redistribute fine grained sediment from the upper reaches of Penrhyn Estuary during storm events. An increased velocity at the eastern end of the new channel (refer to Location D in Figure 16.2) is predicted, but the velocities remain low (less than 0.12 m/s).

Intertidal sand flats in Penrhyn Estuary would be created as part of the habitat enhancement for migratory birds. During a fresh water flood, notably at tides above mean sea level, there is likely to be some flow outside the formed flow path through the Estuary that may result in creation of additional channel(s). The result would be some redistribution of sediment and “wandering” of the channel(s) within Penrhyn Estuary.



15.5 Mitigation Measures

The results of the modelling show that the effects of the proposed Port Botany Expansion on the hydrodynamics and coastal processes within Botany Bay are essentially unchanged. Therefore, no specific mitigation measures are proposed. However, the following provides a summary of the design features which were incorporated into the Port Botany Expansion to minimise potential impacts on the hydrodynamics and coastal processes of the Bay. These include:

sloping rock revetment design for the new terminal to minimise reflection and propagation of waves;

alignment of the dredging profile to minimise potential increases in swell waves along the Parallel Runway and throughout the Bay;

construction of a wide (130 m) channel to ensure adequate flushing of Penrhyn Estuary; and

provision of a rock groyne at the northwestern end of Foreshore Beach to reduce sediment accumulating in the mouth of the Mill Stream.

The hydrodynamic modelling undertaken has been calibrated and verified against recorded measurements in Botany Bay. As such, there is a high level of confidence in the results and predicted impacts from the modelling. Nevertheless, it is recognised that even the most advanced models remain approximations of reality. Consequently, a monitoring programme would be undertaken by Sydney Ports Corporation to measure and compare tidal current flows and beach processes against predicted impacts of the proposal. The monitoring programme would include:

continuous recording of wind and wave climate in Botany Bay and offshore;

beach profiling or aerial photographic record/photogrammetric analysis of Silver Beach, Towra Beach, Spit Island, Lady Robinsons Beach and Foreshore Beach to assess and quantify any significant changes in sandy beaches and nearshore shoals; and

ongoing assessment of the need for removal of accumulated sand at the groyne and any replenishment required at the new boat ramp.

15.6 Conclusion

Results of numerical modelling indicate that currents in Botany Bay would be essentially unchanged as a result of the dredging, reclamation and construction of the new terminal.

Swell waves penetrating the Botany Bay entrance and local sea waves are the predominant waves affecting sediment transport and shoreline stability in Botany Bay. The swell wave climate in Botany Bay would be effectively unchanged as a result of the construction of the new terminal. Potential changes to swell wave energy, notably along the Parallel Runway, have been minimised by careful design of the dredge area and hard surfaces of the new terminal.

Assessment of local sea waves showed that negligible changes to existing conditions would occur in parts of Botany Bay outside the immediate Port Botany area. Any change in longshore transportation rates on the southern shoreline of Botany Bay would be imperceptible. Minor changes to the long wave climate would not be expected to affect sediment transport along the Parallel Runway. The minor changes predicted to the



wave climate in northeastern Botany Bay would not have an adverse effect on ships berthed at the proposed or existing terminals.

Construction of the new terminal would likely increase accumulation of fine grained sediment in Penrhyn Estuary. However, the influx of fine grained sediment from the catchment would be low. Sediment would continue to accumulate near the Mill Stream outlet, but a groyne would be constructed to capture the sand and prevent it from entering the Mill Stream.


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